Heat Treating: Fastener Furnaces 101
Fasteners, commodity or specialty, frequently find their installations in critical applications where an unexpected failure can lead to catastrophic results. As critical as these fasteners are, they still must be heat treated as efficiently as possible. Remember the old joke, "One astronaut turns to the other and says, ‘We're riding on the 1,000,000-pound spaceship that took 10 years to design, costs billions of dollars and was built by the lowest bidder.'" That spaceship, our planes and the cars we drive are dependent on fasteners. Our very lives depend on fasteners that cost only a few cents each.
Heat treaters must meet the most stringent quality standards while adapting at the same time to an environment where manufacturing costs have changed rapidly. One reader commented that in just a few years the high to low ranking of costs to produce his products went from - Labor, Material, Energy to a ranking of Energy, Material, Labor. These changes have significantly increased the cost to produce a fastener. While (as of July 2006) natural gas costs have moderated some, the long-term trend is still up and the cost of chrome-nickel alloy is still at a near all-time high. What all this means is that there is less room for error when investing in new, or operating old, heat-treat equipment.
Per discussions with our readers who operate heat-treating furnaces for fasteners, the factors most critical to their operations are Quality and Efficiency. The first, Quality, is a prerequisite. Quality standards must be met or exceeded if the product is to be shipped. The readers interviewed were most interested in discussing ways to meet the quality requirements as efficiently as possible. In this article we will discuss some of the factors that most influence the selection of the optimum type of heat-treating equipment for your operation.
Part Mixing and NickingThe characteristics of quality include, but are not limited to, the quality and consistency of the surface and/or case, quality and uniformity of the as-quenched condition, limits to distortion, limits to nicking, uniformity of the tempering process, and lastly, part mixing. An adequate furnace system will meet the quality requirements, and an exceptional system meets the requirements while minimizing operating costs.
There are two quality issues that require closer examination - part mixing and part nicking. Most end users interviewed felt these are significant areas of concern, especially for heat treaters that must process a wide range of fasteners and/or are compelled to process smaller lots of fasteners.
Mixing occurs for any number of reasons, some not always apparent, and this has caused many a heat treater a sleepless night.
When processed in a continuous furnace, the most obvious and controllable cause of mixing is the gap between lots. Increase the gap time and the probability of mixing is reduced, but increased gap times increase the cost of operation. Furnace loading systems and part geometry largely determines the length of the gap required to reduce the chance of mixing for a given part, and not all parts are created equal. One respondent suggested that parts be assigned a roll characteristic to define how far the part may travel when loaded on the belt. A better understanding will help users to minimize gaps while providing an adequate level of confidence that no mixing will occur.
Another obvious cause of mixing is operator error, such as failure to change a work tub. This is an area where automation can assist. Many modern furnace lines have systems designed to track the progression of work through the furnace and will either shuttle a new tub into position or accumulate parts until the proper tub is placed under the discharge of the tempering furnace or dip tank. Such systems may initially seem expensive. But compare this cost with the cost of manually sorting thousands of parts, or worse, shipping the mixed part to an automotive assembly line and jamming a screw gun.
Mixing also frequently occurs in the quench. Anyone who has spent any time maintaining continuous equipment has a story of the strangest place they have found a part...sitting on end on top of a heating tube, inside an oil return line, etc. Often the problem is that there is little separation between the minimum oil flow rate that will provide an adequate quench and the maximum flow rate before the parts are blown out of the chute and/or off the return conveyor. Quench tanks are generally constructed with a great deal more flow adjustment than is utilized by furnace operators. The other problem is the flexibility trade-off - the range of part sizes that may be processed and the design of the quenching system. If the quench is optimized for small parts, with extensive shrouding to direct parts and reduce mixing, running large parts may damage key elements, increasing the chance of mixing the next time the small part size is processed.
Another consideration is whether a given part is suitable for a given furnace. You just spent over a million dollars on a new furnace line-only now you have to tell the plant manager that part X should really be sent to another facility for processing. Tough call, but it may be the right call.
Part nicking must be considered when determining any strategy to reduce mixing. Threaded fasteners are prone to nicking (Fig. 2). In general, limiting a part's velocity, and therefore its acceleration, when it comes into contact with another part or any other hard surface reduces nicking. The problem is that a slow-moving part is generally more likely to get hung up as it progresses through a handling system. Most operators of fastener heat-treating equipment have experienced this phenomenon. Lower the angle of the chute to slow down parts before they fall into the tub, and the last few parts end up resting in the chute.
Equipment TypesFasteners are processed in a variety of furnace configurations. Thirty years ago, the most common system configuration was the pre-washer, cast link belt, oil quench, washer and tempering furnace. The cast link belt is efficient and robust. Cast link belts are efficient because the conveyor belt never leaves the furnace. The system is robust because the belt is constructed of interlocking cast pieces (Fig. 4). A properly designed and maintained cast link belt furnace still enjoys the reputation as an excellent design, providing the user with an effective and efficient system.
The primary drawback to most cast link belt furnaces is loading and the resulting problems with part mixing and nicking. For the most part it is difficult to reliably deposit the work on the belt without the part rolling or tumbling forward. Also, there is frequently an increased risk in nicking when loading a cast link belt. Over the years, there have been many different loading system designs that minimize these problems, but frequently it has been easier to solve the problem by moving to a different type of equipment.
While there are still a large number of cast link belt furnaces in operation, new buyers have increasingly looked to mesh belts when deploying new systems. The mesh belt has solved some of the inherent loading problems associated with cast link belts. It is easier to get access to the belt so parts can be loaded at a slower speed to minimize part rolling and tumbling. The main drawback to many early mesh belt designs was that the belt cools as it exits the furnace - making early mesh belts inherently less thermally efficient. Much work has been done to solve this problem, and modern designs are much more efficient. As indicated by the market-share obtained by mesh belt furnaces, the improved loading characteristics far outweigh any increase in energy consumption.
Another option, though rarely purchased, is a rotary retort furnace. There are certain parts that are particularly well suited to a rotary, where the part geometry makes conveying on either a mesh or cast belt very difficult. One serious drawback to this type of furnace is the massive cast or fabricated retort made of chrome-nickel alloy. These retorts are expensive to purchase and replace. Another problem is that the parts bunch as they are discharged into the quench. Limiting damage in the quench can be very challenging.
No matter which type of continuous system is selected, modern controls provide increased repeatability and can help reduce operator error. Most modern systems are available with computer controls that interface with plant-wide systems to ensure the correct part is loaded and processed at the proper temperature and carbon potential. These systems can do wonders to minimize gaps while providing the correct processing environment.
The last option, and one that may be the best choice in some cases, is the integral quench batch furnace. In this furnace, loose parts are loaded in some type of basket or hand loaded into a fixture and bulk quenched. As with any integral quench, the total surface area of the work being processed must be limited to ensure safe and uniform quenching. For manufacturers who must process their work in-house and whose volume is very limited, the Integral Quench may be the furnace of choice. Other fastener applications well suited to batch furnaces are extremely large or long fasteners and/or parts that must be hand loaded to ensure there is no surface damage.
Most systems built in the last 30 years include some type of heat recovery or recuperation. However, these systems were deployed when gas was significantly less expensive. With natural gas averaging $6.00 per decatherm, there may be many additional opportunities to effectively reuse waste heat and improve new and existing system efficiency.
MaintenanceAs with all things, proper maintenance is required to ensure that any system continues to operate. Maintenance should not be an "after the system is purchased" concern. The end user should consider every aspect of maintenance during the design phase. A properly located access door or adequate clearance in a foundation pit may make the difference between a four-hour turn-around to service a key component or clean a quench, and a two-day ordeal.
Equipment Sizing and EfficiencyAs a rule, furnace equipment achieves its optimum efficiency when fully loaded. Unfortunately, just in time, reduced lot sizes, pull systems, etc. - all great concepts to improve overall manufacturing efficiency - make operating a continuous heat-treat system very difficult. These requirements, in the interest of reduced in-process inventory and quicker response time, force the heat treat to process smaller lots. The processing time versus gap time ratio decreases - driving up per unit operating costs. The best way to address these changes is to ensure your systems are properly sized. Unfortunately, heat loss, atmosphere consumption and operating labor are generally fixed. If many small lots are anticipated, it may be better to consider a batch furnace. It may also be desirable to operate a narrow, long belt to reduce the impact of gaps.
A number of smaller furnaces may provide improved flexibility, redundancy and thermal efficiency. Multiple furnaces facilitate maintenance by minimizing the impact of taking a furnace off-line for repair or preventive maintenance.
The Department of Energy, in cooperation with the Industrial Heating Equipment Association, has developed a software tool designed to assess the efficiency of your current system and the effect of improvements. This software package, called the Process Heating Assessment and Survey Tool (PHAST), is available for free and can be downloaded at http://www1.eere.energy.gov/industry/bestpractices/process_heat.html.
Don't assume that a new furnace is too expensive. As energy prices rise over time, the ratio of operating costs versus capital/depreciation costs will likewise continue to rise. While a new furnace system may not be justifiable, it is time to re-evaluate operating costs to see if an investment in new heat-treating technology is needed.
For more information: John B. Clarke is President of Diamond Engineering Company, 3723 W. Hamilton Road, Fort Wayne, IN 46814-9728; tel.: (260) 625-5494; fax: (248) 479-0989; e-mail: firstname.lastname@example.org
Additional related information may be found by searching for these (and other) key words/terms via BNP Media LINX at www.industrialheating.com: fastener furnace, cast link belt, mesh belt, rotary retort furnace, integral quench, thermal efficiency, preventive maintenance